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Nathalie Dubois

Dr.
Nathalie Dubois

Department
Surface Waters Research & Management

About Me

My research spans across the boundaries of sedimentology, paleolimnology, paleoclimatology, inorganic and organic geochemistry. The main reason for this broad spectrum is that I enjoy combining different methods and tools to tackle a question from different angles.

The research projects I have been developing since joining the Eawag in 2013 revolve around two central themes:

(1) The generation of new high-resolution paleoclimatic and paleoenvironmental reconstructions using lake sediments, with a focus on first human impacts on aquatic ecosystems.

(2) The translation of water column processes into sedimentary signatures, in particular through the use of sediment traps to track depth-dependent dissolution and sediment focusing in lakes.

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Curriculum Vitae

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Selected publications

Millennial-scale Atlantic/East Pacific sea surface temperature linkages during the last 100,000 years

Amplifying both internally generated variability and remote climate signals from the Atlantic Ocean via coupled air–sea instabilities, the eastern tropical Pacific (ETP) is well situated to detect past climate changes and variations in Central American wind systems that dynamically link the Atlantic and the Pacific.
Here we compare new and previously published alkenone-based sea surface temperature (SST) reconstructions from diverse environments within the ETP, i.e. the Eastern Pacific Warm Pool (EPWP), the equatorial and the northern Peruvian Upwelling regions over the past 100,000 yr. Over this time period, a fairly constant meridional temperature gradient across the region is observed, indicating similar hydrographic conditions during glacial and interglacial periods. The data further reveal that millennial-scale cold events associated with massive iceberg surges in the North Atlantic (Heinrich events) generate cooling in the ETP from ∼8°N to ∼2°S. Data from Heinrich event 1, however, indicate that the response changes sign south of 2°S. These millennial-scale alterations of the SST pattern across diverse environments of the ETP support previous climate modeling experiments that suggested an Atlantic–Pacific connection caused by the intensification of the Central American gap winds, enhanced upwelling and mixing north of the equator and supported by positive air–sea feedbacks in the eastern tropical Pacific.

Indonesian vegetation response to changes in rainfall seasonality over the past 25,000 years

The hydrologic response to climate forcing in the Indo-Pacific warm pool region has varied spatially over the past 25,000 years1-5. For example, drier conditions are inferred on Java and Borneo for the period following the end of the Last Glacial Maximum, whereas wetter conditions are reconstructed for northwest Australia4. The response of vegetation to these past rainfall variations is poorly constrained. Using a suite of 30 surface marine sediment samples from throughout the Indo-Pacific warm pool, we demonstrate that today the stable isotopic composition of vascular plant fatty acids (δ13Cfa) reflects the regional vegetation composition. This in turn is controlled by the seasonality of rainfall consistent with dry season water stress6. Applying this proxy in a sediment core from offshore northeast Borneo, we show broadly similar vegetation cover during the Last Glacial Maximum and the Holocene, suggesting that, despite generally drier glacial conditions1,7, there was no pronounced dry season. In contrast, δ13Cfa and pollen data from a core off the coast of Sumba indicate an expansion of C4 herbs during the most recent glaciation, implying enhanced aridity and water stress during the dry season. Holocene vegetation trends are also consistent with a response to dry season water stress. We therefore conclude that vegetation in tropical monsoon regions is susceptible to increases in water stress arising from an enhanced seasonality of rainfall, as has occurred8 in past decades.

Presence of oxygen and aerobic communities from sea floor to basement in deep-sea sediments

The depth of oxygen penetration into marine sediments differs considerably from one region to another1, 2. In areas with high rates of microbial respiration, O2 penetrates only millimetres to centimetres into the sediments3, but active anaerobic microbial communities are present in sediments hundreds of metres or more below the sea floor4, 5, 6, 7. In areas with low sedimentary respiration, O2 penetrates much deeper8, 9, 10, 11, 12 but the depth to which microbial communities persist was previously unknown9, 10, 13. The sediments underlying the South Pacific Gyre exhibit extremely low areal rates of respiration9. Here we show that, in this region, microbial cells and aerobic respiration persist through the entire sediment sequence to depths of at least 75 metres below sea floor. Based on the Redfield stoichiometry of dissolved O2 and nitrate, we suggest that net aerobic respiration in these sediments is coupled to oxidation of marine organic matter. We identify a relationship of O2 penetration depth to sedimentation rate and sediment thickness. Extrapolating this relationship, we suggest that oxygen and aerobic communities may occur throughout the entire sediment sequence in 15–44% of the Pacific and 9–37% of the global sea floor. Subduction of the sediment and basalt from these regions is a source of oxidized material to the mantle.